As daylight time grows shorter, and some of us try to synchronize our body clocks with this weekend’s switch to standard time, it’s a good time to smack yourself upside the head so that you stay awake behind the wheel.

The AAA Foundation for Traffic Safety is out with new research that shows that 21 percent of fatal crashes involve tired drivers. That’s a higher percentage than the driver fatigue calculations of the National Highway Traffic Safety Administration (NHTSA).

“This new research further confirms that drowsy driving is a serious traffic safety problem,” said Peter Kissinger, head of the AAA foundation. “Unfortunately, drivers often underestimate this risk and overestimate their ability to combat drowsiness behind the wheel.”

The researchers determined that one third of crashes involving a fatigued driver result in injuries and that more than 6,000 tired driver crashes each year result in at least one death.

“Like distraction, there are limitations in relying on crash-involved drivers to realize, remember and willfully report their level of impairment,” said John B. Townsend II of AAA. “This study leveraged findings from trained crash investigators, as opposed to police reports, as a source of data.”

Young drivers, ages 19-24, are more likely to admit to driving while drowsy, while the oldest drivers (ages 75 up) and the very youngest (ages 16-18) were the least likely to admit to it, earlier AAA research found.

“Despite the fact that 95 percent of Americans deem it ‘unacceptable’ to drive when they are so tired that they have a hard time keeping their eyes open, more than 28 percent admit to doing so in the last month,” Kissinger said.

The report was based on the analysis of 14,268 crashes in 2009-2013 in which at least one vehicle was towed from the scene.